MIPS: AR7: rewrite of cpmac_get_mac()

Shamelessly stealing wisdom from pasemi_mac.c, I found char2hex() could
be replaced with a single call to sscanf(), looks cleaner to me at
least.  The result is 100 bytes trimmed off the size of a compiled
cpmac_get_mac() and as an extra bonus it grumbles and gracefully fails
over to using random_ether_addr() when an attempt to parse an invalid
MAC address is made.

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>
To: linux-mips@linux-mips.org
Cc: florian@openwrt.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>